System and method for pre-stretching plastic material

ABSTRACT

A system and method are disclosed for pre-stretching plastic material that is later used in wrapping a plurality of objects. Plastic material is continuously or non-continuously provided to a pre-stretching device including a plurality of rollers. The plastic material is passed around the plurality of rollers within the pre-stretching device and is then fed through an accumulator to a strapping device. The passing of the plastic material around the plurality of rollers causes the plastic material to stretch by a predetermined amount before entering the strapping device.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. No. 60/673,029, filed Apr. 20, 2005 and herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the manufacture and use of strapping products. More particularly, the present invention relates to the preparation of strapping materials for use in securing items such as plastic containers.

BACKGROUND OF THE INVENTION

The plastic container industry has utilized strapping systems to unitize their products for over thirty years. In the past twenty years, this industry has migrated to plastic strapping systems to unitize their products.

In recent years, the plastic container industry has undergone its own transformation. Plastic containers have become lighter and more cost efficient to manufacture. Additionally, the appearance, shape and design of containers has become a marketing tool. These changes to the plastic containers have created packaging issues for the unitization of plastic bottles.

Historically, the amount of initial strap tension applied to a plastic container skid/unit was great enough to allow for the tension decay typical of plastic strapping, as well as the tension drop resulting from the shifting and shrinkage of individual plastic containers, while maintaining adequate package containment pressure to ensure proper unitization. New plastic container designs and increasing market pressure to produce containers at lower prices (meaning less resin per container), however, have created new challenges for the strapping process. In today's market, new container designs often do not allow for initial strapping tension levels above about 65 lbs. For this reason, the tension levels that are used fall to about 40 lbs. At this level, strapping is unsuitable for maintaining adequate package containment pressure to ensure proper unitization of the plastic containers.

Without an acceptable level of performance from strapping products, additional packaging products have been introduced to assist the packaging of plastic containers. In particular, the use of plastic stretch film has been introduced to aid in packaging. With the introduction of stretch film, the industry found an improved process that provided the level of unitization required to transport their plastic containers to their customer. However, the introduction of stretch film to the unitization process for plastic containers also increased the cost of unitization by a multiple of about 2.5. Therefore, it would be desirable to develop a system and method for unitizing bundles of plastic containers that possess the benefits of stretch film while also lowering the unitization cost.

SUMMARY OF THE INVENTION

The present invention provides for an improved system and method of pre-stretching material for use in unitizing groups of plastic bottles. The present invention involves the use of a novel pre-stretching machine having a plurality of rollers. The rollers are used to pre-stretch the plastic strapping, which is then transferred to an accumulator before being used to package the respective containers.

The pre-stretch system and method of the present invention provides for the elimination of stretch film while producing higher retained strap tension that provides for a secure load of plastic containers. The present invention effectively eliminates the need for high tension of the package while providing a strap that is within the dynamic working range for retained tension. The plastic strapping can be pre-stretched without destroying the properties and characteristic of the plastic strapping, instead only temporarily losing stability and rigidity in the strapping.

These and other objects, advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between strap load vs elongation over time;

FIG. 2 is a side view of a pre-stretching device, a material accumulator and a packaging machine according to one embodiment of the present invention;

FIG. 3 is a front view of pre-stretching device of FIG. 2, along with a representation of a portion of the material accumulator with the position of the accumulator wheel assembly, both according to one embodiment of the invention;

FIG. 4 is a side view of the pre-stretching device of FIG. 3;

FIG. 5 is a top view of the pre-stretching device and material accumulator portion of FIG. 3;

FIG. 6 is a graph illustrating the effect of pre-stretching on retained tension; and

FIG. 7 is a graph illustrating the effect of pre-stretching on stiffness over time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Plastic strapping is produced using a stretching process that is similar to that employed in the production of fibers. This process results in the preferential orientation of polymer molecules (or chains) along the strap length and a several-fold increase in tensile strength.

Oriented plastic exhibits viscoelastic-plastic behavior in response to mechanical loading. Elastic deformation and recovery is partly immediate and partly time-dependent. Yielding occurs over a broad range rather than at a well defined point, and the yield deformation itself is time dependent and only partly permanent (or “plastic”), the rest being recoverable over time. It is believed that the yield corresponds to plastic shearing between polymer chains corresponding to the disruption of secondary inter-chain bonds, for example as discussed in Northolt, M. G. et al., Polyer Vol. 36, No. 18, pp. 3485-3492, 1995, herein incorporated by reference, which provides a good description of the material mechanics and propose relevant theoretical models.

FIG. 1 illustrates how upon loading, unloading and almost immediate reloading, the yield “knee” in the load-elongation curve is no longer apparent. However, with a sufficient delay prior to reloading, the original yield characteristics return as the material seeks an equilibrium and secondary inter-chain bonds are re-established.

Being viscoelastic, plastic strapping is susceptible to time dependent relaxation upon loading. Plastic strap that is loaded and held at a constant elongation will undergo a load decay (stress relaxation) while plastic strap that is under constant load will continue to elongate over time (creep). This behavior contrasts with that of steel, for example, where creep and stress relaxation are negligible.

As previously mentioned, stress relaxation can compromise package unitization. It has been recognized that fibers which exhibit viscoelastic effects such as stress relaxation could be diminished by mechanical conditioning consisting of pre-loading the fibers to a load level that is higher than that used in the second loading. This is discussed, for example, in Leaderman, H., Elastic and Creep Properties of Filamentous Materials and Other High Polymers, 1943, herein incorporated by reference. The present invention makes use of this technique, hereafter referred to as “pre-stretching”, to reduce the strap tension losses that result from stress relaxation and package settling.

FIGS. 2-5 show a pre-stretching device 100 and a material accumulator 200 constructed according to one embodiment of the present invention. As shown in FIGS. 2-5, the pre-stretching device 100 includes a bevel gear motor 110 that is used to provide power to a plurality of rollers 120. Although a bevel gear motor 110 is depicted in FIGS. 2-5, other types of motors may be used. A pair of polychains 130 are used to transfer the energy generated by the bevel gear motor 110 to the plurality of rollers 120.

In the embodiment of the invention depicted in FIGS. 2-5, ten of the rollers 120 are included in the pre-stretching device 100, with the rollers 120 separated into a first roller group 140 and a second roller group 150.

The path of material 160 from a coil 165 to be pre-stretched is shown in FIGS. 2 and 3. The material 160, which can comprise polyester or polypropylene strapping in various embodiments of the invention, is fed from the coil 165 through an entrance point 170 on the pre-stretching device 100. In one embodiment of the invention, the material 160 passes over a preliminary tower 185 before reaching the entrance point 170. After passing through the entrance point 170, the material 160 passes around each of the rollers 120 in the first roller group 140. Once the material 160 has passed around the last of the rollers 120 in the first roller group 140, it travels to the second roller group 150, where it passes around each of the remaining rollers 120 before exiting the pre-stretching device 100 through an exit point 180. In one embodiment of the invention, the first roller group 140 and the second roller group 150 each comprise five of the rollers 120.

Generally, the material 160 can be pre-stretched to any elongation level that exceeds that which is applied to the package, but that is less than the break elongation of the strap. In one embodiment, for polyester strapping, the pre-stretch elongation level should be at least about 5% in order to realize a benefit to unitization and it should be no greater than about 10% so as to minimize the risk of the strap fracturing during pre-stretching. Within this range, higher pre-stretching levels offer greater potential benefits to unitization, but at the cost of greater reductions to flexural rigidity and the resulting challenges to consistent feeding of the strap through large arches.

FIG. 7 illustrates the effect of pre-stretching on stiffness of the strap. As can be seen from FIG. 7, pre-stretching results in a lower bending load being necessary to achieve a particular deflection angle. The impact of the prestreching on stiffness dissipates over time, as can be seen from the increase in stiffness from the 0.5 minute sample through to the 19 hour sample. In addition, a greater amount of pre-stretching will result in a greater reduction in stiffness as can be seen in the 10% pre-stretching sample in comparison to the non-stretched and the 7% pre-stretch samples.

In one embodiment of the invention, the ratio of the roll surface speed of the second roller group 150 to that of the first roller group 140 is fixed at about 1.07:1. This causes the material 160 to be stretched by up to about seven percent over its original length. Importantly, this design maintains the original properties and characteristics of the material 160 and maintains the integrity of the material 160 due to the recovery of the pre-stretched strapping, ultimately creating a tighter fit around the objects to be strapped during the unitization process. This embodiment of the present invention may be used in conjunction with a commercial strapping system to achieve improvement tension retention on PET bottle loads, as shown in FIG. 3. It may be seen that one week following strap application, the tension loss in the pre-stretched straps was two thirds that of the straps that were not pre-stretched.

In one embodiment of the invention, each of the plurality of rollers 120 is preferably coated with urethane rubber. The urethane rubber coating provides a high friction surface which prevents the material 160 from slipping on the rollers 120. It will be evident to those skilled in the art that slippage could also be prevented by using other types of surface finish, or by increasing the wrap angle or the number of rollers, or perhaps by pre-stretching in a plurality of stages.

The material 160 can be fed through the pre-stretching device 100 at a variety of speeds. In one embodiment of the invention, the material 160 is routed through the pre-stretching device 100 at about five feet per second. In another embodiment, the material 160 is routed through the pre-stretching device 100 at about four feet per second. At this speed, the material 160 is stretched by up to about eight percent over its original length. Importantly, this design maintains the original properties and characteristics of the material 160 and maintains the integrity of the material 160 due to the recovery of the pre stretch strapping, ultimately creating a tighter fit around the objects to be strapped during the unitization process.

After the material 160 exits the pre-stretching device 100 through the exit point 180, it enters the material accumulator 200 shown in FIGS. 2, 3 and 5. The material accumulator 200 includes a housing 210. The material accumulator 200 includes an accumulator entrance region 230 for the material 160. Substantially opposite the entrance region 230 is an exit region 240. The exit region 240 includes an accumulator wheel assembly 265 which, in one embodiment of the invention, includes a wheel bracket 250 for mounting a flat accumulator wheel 260 and a flanged accumulator wheel 270. The material 160 passes through the accumulator entrance region 230, across the material accumulator 200, and through the accumulator exit region 240 through the accumulator wheel assembly 265, particularly the flat accumulator wheel 260 and the flanged accumulator wheel 270. The flanges on the flanged accumulator wheel 270 act as a guide for the material 160.

In one embodiment of the invention, the accumulator wheel assembly 265 is positionally adjustable. For example and as shown in FIG. 2, the accumulator wheel assembly 265 is located at the top of the material accumulator 200. In FIG. 3, on the other hand, the accumulator wheel assembly 265 is positioned closer to the bottom of the material accumulator 200. The relative height of the accumulator wheel assembly 265 has a direct effect on the speed at which the material 160 passes through the pre-stretching device 100 and the material accumulator 200. In particular, and in one embodiment of the invention, the material 160 moves at a maximum speed when the accumulator wheel assembly 265 is located near the bottom of the material accumulator 200, with the accumulator wheel assembly 265 moving at progressively lower speeds as the accumulator wheel assembly 265 is raised.

After exiting the material accumulator 200 though the accumulator exit region 240, the material 160 is routed to a conventional strapping system 300, as shown in FIG. 2.

In another embodiment of the invention (not shown), a non-continuous strap feed can be used. This embodiment allows for the momentary control of pre-stretch material 160 using an accumulator to “hold” the material 160 as it waits to move into active feeding around a skid of objects to be unitized.

The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. 

1. A method of pre-stretching a plastic material for use in unitizing a plurality of objects, comprising: providing the plastic material to a pre-stretching device including a plurality of rollers; passing the plastic material around the plurality of rollers within the pre-stretching device; and feeding the plastic material from the pre-stretching device to a strapping device, wherein the passing of the plastic material around the plurality of rollers cause the plastic material to stretch by a predetermined amount before entering the strapping device.
 2. The method of claim 1, wherein the plurality of rollers comprise two groups of rollers.
 3. The method of claim 2, wherein each of the two groups of rollers comprise five rollers.
 4. The method of claim 1, wherein each of the plurality of rollers include a urethane coating.
 5. The method of claim 1, further comprising, before feeding the plastic material to the strapping device, passing the plastic material through an accumulator wheel assembly within an accumulator.
 6. The method of claim 5, wherein the accumulator wheel assembly comprises: an accumulator wheel bracket; a flat accumulator wheel rotatably connected to the accumulator wheel bracket; and a flanged accumulator wheel rotatably connected to the accumulator wheel bracket and positioned relative to the flat accumulator wheel such that the flanged accumulator wheel and the flat accumulator wheel cooperate to guide the plastic material through the accumulator wheel assembly.
 7. The method of claim 5, wherein the position of the accumulator wheel assembly is adjustable relative to the remainder of the accumulator, and wherein the relative position of the accumulator wheel assembly affects the rate at which the plastic material enters the strapping device.
 8. The method of claim 1, wherein the plastic material is passed around the plurality of rollers at about four feet per second.
 9. The method of claim 1, wherein the plastic material is passed around the plurality of rollers at about five feet per second.
 10. The method of claim 1, wherein the plastic material is stretched about five to about ten percent over its original length as it passes through the pre-stretching device.
 11. The method of claim 1, wherein the plastic material is stretched about eight percent over its original length as it passes through the pre-stretching device.
 12. The method of claim 1, wherein the plastic material is stretched about seven percent over its original length as it passes through the pre-stretching device.
 13. An assembly for pre-stretching a plastic material for use in unitizing a plurality of objects, comprising: a pre-stretching device including: a pre-stretching device entrance region, a pre-stretching device exit region, a plurality of rollers positioned between the pre-stretching device entrance region and the pre-stretching device exit region, and an energy and torque source for driving each of the plurality of rollers at a predetermined rate, wherein the plurality of rollers are positioned to define a pathway for the plastic material through the pre-stretching device, and wherein the plastic material is stretched by a predetermined amount as it passes through the assembly before being fed to a strapping device.
 14. The assembly of claim 13, wherein the plurality of rollers comprise two groups of rollers.
 15. The assembly of claim 14, wherein each of the two groups of rollers comprise five rollers.
 16. The assembly of claim 13, wherein each of the plurality of rollers include a urethane coating.
 17. The assembly of claim 13, further comprising an accumulator positioned downstream from the pre-stretching device, wherein the plastic material passes through an accumulator before reaching the strapping device.
 18. The assembly of claim 17, wherein the accumulator includes an accumulator wheel assembly comprising: an accumulator wheel bracket; a flat accumulator wheel rotatably connected to the accumulator wheel bracket; and a flanged accumulator wheel rotatably connected to the accumulator wheel bracket and positioned relative to the flat accumulator wheel such that the flanged accumulator wheel and the flat accumulator wheel cooperate to guide the plastic material through the accumulator wheel assembly.
 19. The assembly of claim 18, wherein the position of the accumulator wheel assembly is adjustable relative to the remainder of the accumulator, and wherein the relative position of the accumulator wheel assembly affects the rate at which the plastic material enters the strapping device.
 20. The assembly of claim 13, wherein the assembly is configured to stretch the plastic material between about five and ten percent over its original length as it passes through the assembly.
 21. The assembly of claim 13, wherein the assembly is configured to stretch the plastic material about seven percent over its original length as it passes through the assembly.
 22. The assembly of claim 13, wherein the assembly is configured to pass the plastic material through the assembly at a rate of about four feet per second.
 23. The assembly of claim 12, further comprising a preliminary tower positioned upstream of the pre-stretching device, the preliminary tower guiding the plastic material upstream of the pre-stretching device.
 24. The assembly of claim 13, wherein the energy and torque source comprises a bevel gear motor.
 25. The assembly of claim 13, wherein the assembly is configured to stretch the plastic material between about eight percent over its original length as it passes through the assembly. 